Aircraft are used in a wide variety of applications, including travel, transportation, fire fighting, surveillance and combat. Various aircraft have been designed to fill the wide array of functional roles defined by these applications. Included among these aircraft are balloons, dirigibles, traditional fixed wing aircraft, flying wings and helicopters.
One functional role that a few aircraft have been designed to fill is that of a high altitude platform. Operating from high, suborbital altitudes, such aircraft can monitor weather patterns, conduct atmospheric research and surveil a wide variety of subjects. Most of these remarkable aircraft have limited flight duration due to fuel limitations. However, a number of aircraft have been proposed that are solar powered, and that can sustain continuous flight for as long as sunlight is available, or even longer.
Three such aircraft that have been constructed are the well-known Pathfinder, Centurion and Helios aircraft, which have set numerous flight records. The basic design underlying these aircraft is discussed at length in U.S. Pat. No. 5,810,284, which is directed toward an unswept flying wing aircraft having a very high aspect ratio and a relatively constant chord and airfoil. While these aircraft are quite noteworthy for their long term flight potential, they do have limits in their available power and payload.
Such aircraft are designed as flying wings that include a number of self-sufficient wing sections, each having one or more electric motors that are driven by power generated in solar cells mounted in that section, and each generating enough lift to support its own weight. To minimize weight, the aircraft structure is highly flexible, and is designed to withstand only relatively small torsional loads along its lateral axis. The aircraft's wing has little or no dihedral while on the ground. However, due to the high flexibility, the large aspect ratio and the constant chord, in-flight wing loads tend to cause the wing to develop a substantial dihedral angle at the wingtips.
To minimize the torsional loads, the aircraft wing includes elevators along a substantial portion of its trailing edge (i.e., the trailing edge of the flying wing). The aircraft does not include a rudder or ailerons, and the elevators are not designed as elevons (i.e., they cannot move in contrary directions near opposite wingtips). Instead, the aircraft turns (and otherwise controls yaw) by using variable thrust applied across the wingspan through the application of different power levels to different motors. Roll is passively controlled by the dihedral of the wing, which is developed in flight. Sideslip is also passively controlled, both by the dihedral of the wing, and by fins that extend down from a number of wing segments in a direction normal to plane of the wing at the fin's span-wise location.
Long duration high altitude platforms that operate at suborbital altitudes, such as the Pathfinder and Centurion aircraft, have been suggested for use in a variety of functions. As one example, a high altitude platform equipped with microwave communications equipment could provide communication relay services between remote areas. In another example, high altitude platforms could measure and study winds, storms or pollutants in the atmosphere. Similarly, governments could use these aircraft to monitor troop movements or narcotics production. Other types of aircraft are not optimally suited to these tasks, because they are limited by the amount of combustible fuels that they use, which are heavy, expensive and are consumed very quickly. Typically, these other types of aircraft cannot remain over their desired location for any significant length of time, and hence, are of limited utility in performing these tasks.
One way around these operational limitations is to use satellites as high altitude platforms. However, satellites are expensive to launch, and typically remain in a permanent, fixed orbit. Some satellites can change their orbit to a limited degree; however, this is done only with great difficulty and expense, and there is a fuel limit to how many orbital changes a satellite may make. For example, if it is desired to measure and study a hurricane that originates in Africa and travels toward the Gulf coast of the United States, satellites cannot, practically-speaking, be asked to follow and track such a storm.
The use of satellites is also disadvantageous for many types of measurement and surveillance as well, because satellites orbit outside the Earth's atmosphere. That is to say, satellites as a practical matter cannot use many tools that optimally require contact with the atmosphere. Photographic images taken by a satellite are also sometimes less than optimal, since the target is usually a great distance from the satellite. Finally, satellites are not easily brought back to Earth and retrieved, e.g., for servicing, and so are typically used only for one very expensive, special purpose task.
Given the broad range of functions that a long-duration, suborbital platform has the potential to perform, it is desirable to design such platforms to be capable of handling larger payloads and power demands. The platforms could be variations of existing platforms, such as the Pathfinder and Centurion aircraft, or they could be newly designed, high altitude platforms.
Likewise, given a high altitude platform with expanded payload and power capabilities, it is desirable to find new uses for the platform. Such new uses can increase demand for the aircraft, and thereby cause increased production and lower production costs. Naturally, new uses also have the potential for new advantages for the public.
In sum, there exists a definite need for a multi-purpose aircraft that can remain airborne for long durations without the need to re-fuel. Preferably, such an aircraft should be able to operate up to very high, suborbital altitudes. Importantly, it is desirable for such an aircraft to have the capability for larger payloads and/or power supply requirements. Furthermore, there exists a need for such an aircraft to be inexpensive to build and operate and, furthermore, pollution-free. Also, a definite need exists for such an aircraft to be able to perform surveillance, testing and measurement functions while being steerable, mobile, and able to perform varying missions of extended duration. Finally, with the availability of the hardware of the present invention, it should be noted that a broad variety of communications needs exist that such a high altitude platform can fill. Various embodiments of the present invention can meet some or all of these needs, and provide further, related advantages.